Hi there. I would certainly say that the most likely diagnosis is worms in this case, as most other causes of diarrhea are infective, and would probably be effecting other birds, this bird maybe didn't get the full dose as you suggested, this can lead to a build up of parasites in the individual bird.
My advise would be to use a product called 'Flubenvet' (Flubendazole) to worm them with, this is given daily for 1 week, and you can obtain it pre mixed in the feed, or as a powder to administer yourself(i find it good to put a small amount on a grape and give them all a grape each for 7 days- easier with less chucks!!)
If you worm them with this every 6 months you should solve most parasite issues, although obviously try not to over graze a particular area for too long or reinfection may occur.
Andrea
Bird Veterinarian
15 years in companion animal practise, BVM&S degree Edinburgh university